1. Field of the Invention
The present invention relates to an apparatus for control of resonance in a brake intervention traction control system.
2. Discussion of the Background
Brake intervention traction control is concerned primarily with providing a transfer of torque between driven wheels through a differential by braking the slipping wheel. Such a traction control (TC) system may be implemented in association with an anti-lock brake system (ABS). A schematic representation of the brake arrangement of a combined brake intervention traction control and anti-lock brake system is shown in FIG. 1 of the accompanying drawings, the system is only illustrated with respect to one wheel for clarity. The system comprises a master cylinder 2 arranged to supply fluid, in response to depression of a driver operated brake pedal 3, to a brake 4 of a wheel 5 via a flow control valve 6 and a first electrically operable solenoid valve 8. A series combination of a second electrically operable solenoid valve 10 and a pump 12 are connected in parallel with the flow control valve 6. The pump 12 is arranged to pump fluid towards the junction of the flow control valve and the first solenoid valve 8. A fluid reservoir 14 is connected to an inlet of the pump 12 via a third electrically operable solenoid valve 16.
During ABS operation the first valve 8 is open and the third valve 16 is closed. Pressure from the master cylinder 2 is applied to the brake 4 via the flow control valve 6. When the wheel 5 starts to skid, the second solenoid valve 10 is operated to allow a decrease in brake fluid pressure at the brake 4, thereby reducing the braking force applied to the wheel 5 and halting the skid. The brake fluid is pumped back to the master cylinder 2 to maintain the fluid volume in the master cylinder (otherwise the brake pedal 3 would sink to the floor). When the wheel 5 has stopped skidding, the second solenoid valve 10 is closed to allow fluid pressure to be reapplied to the brake 4.
During traction control operation, when one or more wheels start to spin under acceleration, the first valve 8 is closed and the third valve 16 is opened. The pump 12 is run so as to apply brake pressure to the brake 4 so as to reduce the drive torque applied through the spinning wheel 5. The fluid pressure is modulated by pulsing the second solenoid valve 10 so as to control the braking force applied by the brake 4. The action of a differential gear (not shown) provided to supply drive to wheels on opposite sides of a vehicle but on a common axle, provides a transfer of torque from the braked drive wheel to the unbraked (non-spinning) drive wheel, thereby enhancing acceleration.
A vehicle can suffer wheel vibration under acceleration due to factors such as compliance of the vehicle's suspension. The vibration may be exacerbated by traction control systems which aim to control the speed of a spinning wheel to slightly above the vehicle speed. During such control, the wheel alternately grips and slips, which may make the vibration worse, especially if the frequency of gripping and slipping is substantially coincident with a resonant frequency of the vehicle suspension. The resonance may be felt by the vehicle occupants as a disturbing juddering. Furthermore, the resonance may cause accelerated wear of suspension and transmission components.